Clothes simulation

Currently, clothes simulation is limited to texture mapping inside Virtual Environ-ments. Of course, it limits the kind of clothes that can be used, moreover the textured clothes are deforming according to the skin surface of the body. It does not reproduce the behavior of the clothes. For example, not wrinkles appear. In a step towards uni-fying cloth simulation to the wonderful universe of Virtual Reality and dreaming about a world where virtual humans could manipulate cloth in real time and in a way that seems so natural for us, real humans, we present a contribution for a fast and robust cloth model suited for interactive virtual cloth design and simulation system.

Literature now brings us several techniques for cloth simulation. Many of them present physically based models for simulating in a realistic way fabric pieces based on elastic deformation and collision response. The first of them used simple mechani-cally-based models, such as relaxation schemes, for simulating objects such as flags or curtains ^{42 43}. More general elastic models were developed for simulating a wide range of deformable objects, including cloth ^{44 45}. Recently, several particle system based models attempted to simulate simple cloth object realistically using experi-mental fabric deformation data ^{46 47}. These models claim to be fast and flexible, as

opposed to finite element models ⁴⁸⁴⁹⁵⁰, which are very accurate, but slow and com-plex to use in situations where behavior models are complicated and where collisions create non-linearity and complex boundary conditions, thus not suited for interactive applications.

Dressing a virtual body is a complex application for these models. It involves the ability to design complex garment shapes, as well as a complex simulation system able to detect and to handle multiple collisions generated between the cloth and the body. Our work contributed to the development and evolution of this topic through several contributions ⁵¹⁵². More recently, we studied how to consider cloth as being an object that could be considered independently from the body which wears it, in-volving the issues of folding, wrinkling and crumpling, with all the associated prob-lems related to collision detection and response⁵³. Our work was materialized by sev-eral garment design and dressing systems for animated virtual actors⁵⁴⁵⁵.

On the other hand, new V.R. technologies and efficient hardware open a very at-tractive perspective for developing interactive systems where virtual actors would interact autonomously with mechanically animated objects, such as the garment they themselves wear. In a nearer goal, we could take advantage of these new tools for interactively designing garments and dressing virtual actors in ways that are much more natural and close to the “real” way of manipulating fabric.

We provide simulation tools to take a step towards the requirements defined above.

Of course, the main problems for interactive or real time mechanical simulation are related to computation speed issues. We should not however trade away design flexi-bility and mechanical modelisation accuracy that would lead to unrealistic cloth simulation. Thus, we describe here a mechanical model that allows to modelise elastic behavior of cloth surfaces discretized into irregular triangle meshes, and which is not much more complicated to a simple spring-mass modelisation. This approach com-bines flexibility ⁵³ with simulation speeds ⁴⁷⁵⁶ which are restricted to regular meshes.

Furthermore, a suited integration method has been associated to this model to maxi-mize simulation timesteps and computation speeds without trading away mechanical stability, which is ensured in a very robust way, compatible with all the inaccuracies resulting from most tracking devices used in 3D positioning and V.R. devices.

Beside this, a new approach for handling geometrical and kinematical constraints (such as collision effects or “elastics”), generalization of the collision response proc-ess ⁵³, ensures collision response as well as integration of different manipulation tools in a robust way that does not alter simulation efficiency and thus makes this system efficient for interactive purposes.

We illustrate the achievements brought by these techniques with the help of exam-ples concerning cloth manipulation, dressing and realtime interactive manipulation.

Cloth Simulation and Animation

Cloth simulation algorithms have been extensively studied in the state-of-art re-search. Current applications allow to design simple garments, to fit them onto a vir-tual body and to make simple animations. The key issue is now to implement such algorithms into efficient and useful systems that will fully take advantage of the new possibilities brought by these techniques.

Several new problematic arise from integrating cloth simulation algorithms in such systems. First, performance and robustness have to be high enough to provide robust systems that will enable a user to generate quality work in a minimum of effort and time. The main algorithms concerning the simulation aspect of cloth animation are the mechanical model, the numerical simulation algorithms, the collision detection algo-rithms and the garment design process.

XX.7.1.1. Mechanical Model

The objective is to build a mechanical modellisation of the cloth behavior that combines adequate realism for representing realistic cloth deformation with computa-tion speed requirements compatible with interactive applicacomputa-tions. The designed ap-proaches should be general enough to be implemented in general contexts where the cloth is represented by a triangle mesh structure, possibly irregular that can be dy-namically updated and transformed.

Several approaches are defined, ranking from full elasticity evaluation within the mesh triangles ⁵³ which aims to perform slow and but realistic modellisation of the fabric behavior using linear viscoelasticity, to faster models that use geometrical ap-proximations to derive a simple spring-mass model into a more realistic model simu-lating realistically the basic elasticity properties of the fabric⁵⁷. The latter approach is successfully applied in interactive garment systems allowing dynamic manipulation of the cloth elements, such as displacements, cutting and seaming.

XX.7.1.2. Numerical Simulation

Our systems mainly use fast numerical integration of particle-system representa-tions of the mechanical models. An efficient Runge-Kutta model⁵⁸ has been adapted to provide optimal timestep control through numerical error evaluation. This approach provides a virtually unbreakable model, that can cope with high deformations as well as strong mechanical parameters variations without exhibiting numerical instabilities.

These features are important for all the interactive applications.

Still, the stiffness of the simulation model is the major performance-limiting prob-lem. The timestep has to be kept small in order to prevent local vibration instabilities to appear, which highly affects simulations with refined meshes and simulations with rigid materials.

XX.7.1.3. Collision Detection

Another important and time-limiting aspect of the cloth simulation problem is col-lision detection. As a cloth and the underlying body are discretized into a huge num-ber of polygons, the problem is to determine efficiently which polygons are in geo-metrical contact, in order to apply reaction and friction effects.

The problem is solved efficiently by using an adapted hierarchical scheme⁵⁹, which is specially adapted for dealing with the self-collision problem, and then extended by orientation-correction algorithms for increased robustness⁵³.